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Wow, better than 3 times the current? That's a lot and sounds like something isn't reading right somewhere. Is there any way to get an actual bench meter or even a clamp-on to check and see what your actual current being used is. I've been using a TrippLite dedicated MSW on my KitchenAid household refer since I got the Bus (61,000 miles ago), but haven't done an actual current draw measurement. Just know we boondock a lot and I have not been having battery problems or excessive draw down's overnight. Can't tell daytime as we have 840W of solar on the roof.

Has anyone ever measured with a Kill A Watt meter on a MSW vs shore power? It is just hard to believe that it would be this far off, but like Pigman1 said, it's more than three time the power being used??

The only thing I can think of is the Kill A Watt can not accurately measure a modified sine wave. But this is really hard to believe!

Does anyone notice extremely heavy draw when using a MSW inverter with anything? Mine is a MSW Xantrex 1500 watt, and has always worked fine. Other than the overhead for the inverter itself, the amp draw of any 120 volt appliance should be the same on shore power or inverter, right?

Can anyone tell me if a Kill A Watt meter will work the same on a modified sine wave inverter as on shore power?

I am asking because I am testing our new residential refrigerator, and I am reading .5 amps when plugged in to electric, and 1.7 amps when on the inverter.

The rest of the readings are the same on both. 120v, 60 Hz.
I already know that some refrigerators do not play well with MSW inverters.

I think the important question is what is the DC power being used being used by the refrigerator. If you can read DC amps being used then you can determine the refrigerator use by looking at DC Amps with refrigerator on and with it off, and the difference is what the fridge uses. Multiply this by 12.5 to get watts. Then you can compare that to power used on normal AC ( 60 Watts ). This is the best way to see what the "real" power use when running from the inverter is.

If you do see 2 - 3 times the power being used when the fridge is being driven by the modified sine wave inverter, then this bad - most likely the non 60 HZ power components of the modified sine wave output are turning into heat somewhere most likely inside the refrigerator compressor and may lead to premature failure of it.

I think the important question is what is the DC power being used being used by the refrigerator. Then you can compare that to power used on normal AC ( 60 Watts ). It is a pretty sure bet that the 1.7 is not accurate.

That is a great idea! I have a Tri-Metric wired in, so I will measure the difference between the refrigerator on and off while using the inverter. Why didn't I think of that??

I can readily see a 3x difference IF the Kill-O-Watt algorithm doesn't take MSW in account. There are a dozen ways to calculate power usage and MSW can mess up several of them. Just like digital clocks typically run at the wrong time (speed) with MSW, a KOW can make mistakes too. As a low cost, consumer grade, device, they probably aren't designed to handle anything but standard utility electric power.

That is well within the current capacity of most of the True RMS DMM's on the market. Tap the line and put in a DMM that measures true RMS and compare that to the reading the Kill a Watt gives.

The basic issue is that a true RMS meter actually slices the current wave form in many slices, calculates the power in each one and add that up to give you a loaded average for the wave form applied. Before the integrated circuits that do that were developed the choice was a device that measured the heat generated in a sampling resistor. Other meter types read the peak of the power applied and scale it as if it was a sine wave.

The MSW is a square wave, with typically higher voltage spikes on the leading edge of each wave as the transistors are switching. You obviously know that the PSW is a nice smooth sine wave curve. When I was a brandy new EE 40+ years ago I worked at company that made precision DC to DC converters and timers for missiles. Same process as the MSW, we just rectified it and made it DC at the final step. That just tells you how old the MSW technology is..................

You have two unknowns here. You don't know how well the kill-o-watt measures correctly a square wave, and you don't know how much the refrigerator is consuming on a square wave.

Yep, I certainly am thinking of installing a PSW dedicated to the Refrigerator, but I am now very puzzled by the Kill A Watt readings.

I designed my whole solar set up using the Kill A Watt and my Tri-Metric. Both have been great tools for my purposes. I just never came across a situation where I measured on the inverter vs commercial power. So now, I am very curious.

I think I will try it with my toaster, on and off the MSW, and see if I get different readings. Then, maybe some other appliances too.

Yep, I certainly am thinking of installing a PSW dedicated to the Refrigerator, but I am now very puzzled by the Kill A Watt readings.

I designed my whole solar set up using the Kill A Watt and my Tri-Metric. Both have been great tools for my purposes. I just never came across a situation where I measured on the inverter vs commercial power. So now, I am very curious.

I think I will try it with my toaster, on and off the MSW, and see if I get different readings. Then, maybe some other appliances too.

The toaster is a simple resistive load vs. the refrigerator compressor which is a AC induction motor with a complex inductive load so do not be surprised if the difference in Kill A Watt readings comparing the toaster between sine wave shore power and the MSW inverter power is totally different ratio than what you see with the residential refrigerator. The toaster won't care what the frequency content of the power is because its just making pure heat. The induction motor is tuned to work best with a pure 60 Hz power source and was not designed to work with something else like MSW that is not pure 60 Hz power. Does not mean it won't work with MSW but it is not a sure thing.